In many systems which utilize fluids, such as hydraulic fluids and the like, it is frequently desirable or necessary to monitor and control fluid contamination. This is typically carried out by the controlled passing of a sample of the fluid through a sensor unit which is a part of an automatic particle counter (APC) system. Many of these sensor units, as for example the Royco Model 325 APC sensor unit, are constructed so that the test fluid is passed through a restricted or narrow passage, frequently referred to as a sensing slot or sensing window. The sensor unit operates by sensing the amount of light that is blocked when a particle enters the sensing window fluid passage, where a light source on one side of the window is axially aligned with a light sensor element on the opposite side of the window.
In testing or calibrating the above type of APC, it would of course be highly desirable to use a calibration process and apparatus which is of maximum efficiency and accuracy while at the same time being as simple and economic as possible to provide several prior art processes for calibrating APCs depended on the use of a sample test fluid containing a known quantity of particles of predetermined size. Such calibration and testing systems and procedures typically depended on the use of precision-sized microspheres, which can be quite expensive. Such testing procedures are also typically disadvantaged because of other considerations such as test sphere availability and handling problems, long calibration times, accuracy problems, and problems relating to maintaining suitable cleanliness of the test area and preventing extraneous contamination.
One prior art approach to calibrating a particle counting apparatus using a sample test fluid approach similar to the aforementioned testing procedures is disclosed in U.S. Pat. No. 3,412,037 to Gochman et al. The APC unit that is to be calibrated operates on a flow of diluted blood that is passed through a flow cell of small cross-section. Illuminated optical means are coupled to the flow-cell and detect the passage of individual blood cells therethrough, providing an output pulse signal in response to the passage of the blood cells. U.S. Pat. No. 3,412,037 is primarily directed to the provision of a standard fluid which approximates blood cells in blood, and which can then be used to calibrate the APC. The standard comprises a suspension of minute unicellular fungi in a solution of sodium chloride in a concentration of 5 to 15%, and sodium flouride in a concentration of one half of 1% or greater.
A somewhat different approach to calibration, which is utilized to calibrate a particle velocity measuring instrument rather than an APC, is disclosed in U.S. Pat. No. 3,885,415 to Burns et al. In the U.S. Pat. No. 3,885,415 calibrating system, a movable rotatable calibration disc is generally disposed and moved within and through the optical viewing path of an analytical system devoted to calculating particle related information. A predetermined number of contrast regions representing particles are located upon the surface of the disc, and when the disc is rotated at a given rate within the optical viewing path it simulates predetermined particle movement enabling calibration of the analytical devices. The U.S. Pat. No. 3,885,415 calibrating system calibrates fluid borne colloidal suspension velocities, and is not concerned with the size measurement or quantity of particulate matter in a fluid.
Another calibration system which utilizes a rotating disc is disclosed in U.S. Pat. No. 3,127,464 to Gustavson. In this patent, the aerosol particle counter that is being calibrated is designed to sense light reflected from particles in a target area. The device attempts to simulate the reflections by transmitting portions of the light beam through apertures of known sizes, then redirecting them to a sensor. The system only simulates a calibration, and assumes that all particles of equal size will have equal reflective characteristics regardless of the shape, texture, or color. It is intended that any device using the method of U.S. Pat. No. 3,127,464 be configured so the path of the reflected light is at an angle to the path of light from the light source, so that the paths are not in axial alignment.
The APC device to be calibrated by the calibration system and device of the present invention is not a particle velocity measuring instrument as in U.S. Pat. No. 3,885,415, and is not a simulation system as in U.S. Pat. No. 3,127,464. The APC systems which are to be calibrated in accordance with the present invention sense the amount of light that is blocked when a particle enters an optically monitored reduced cross-section sensing slot or window, where the light source and sensor are in axial alignment. Rather than simulating reflections as in U.S. Pat. No. 3,127,464 or simulating contrast areas moving with known velocity as in U.S. Pat. No. 3,885,415, the APC of the present invention is calibrated by the placing of a single opaque spot in the path of a light beam whose optical system is in axial alignment, blocking the exact amount of the light as a particle with the same cross-sectional area. This enables an absolute and true sizing calibration of the APC. In addition, neither of the discs in U.S. Pat. Nos. 3,885,415 and 3,127,464 is adaptable for use in the sensing slots or windows of the sensor units of the APCs being calibrated in accordance with the present invention.
In view of the problems and requirements to be met in calibrating APC devices having optical type sensors which operate with reduced cross-section sensing windows, and the desire for less costly, time consuming and complex calibrations systems, there is a need for a simplified, economical, fast and efficient (accurate) sizing and calibration apparatus and process for calibration of automatic particle counters. Ideally, the calibration system and process should be readily adaptable to a relatively non-complex testing environment and to a variety of different models of Automatic Particle Counters available on the market today.
From the foregoing, it can be seen that it is a primary object of the present invention to provide a calibration system and process for one in calibrating APCs of the type having sensor units which utilize reduced cross-section sensing windows for the passage of test fluids.
It is also an object of this invention to provide a calibration system which is economic to provide and operate, and which is efficient and precise in calibrating various APC models.
It is another object of the present invention to provide calibration system and process for calibration of APCs, which can be utilized in a conveniently operated manner without time consuming, costly, and complex operations directed at maintaining the cleanliness of the testing area, and which minimizes cleanliness and contamination problems normally associated with APC calibration operations.
It is yet another object of the present invention to provide a system and process for calibration of APCs which eliminates the need for precision-sized and shaped spheres and particles.
It is another object of this invention to provide an apparatus that enables fast and precise calibration of the electronic and optical components of standard APC units.